This invention relates to digital circuitry which discriminates against contact bounce emanating from a subscriber terminal in a digital telephone switching system and more particularly to apparatus for providing debouncing for a plurality of multiplex subscriber line circuits.
The telephone switching system is a prime example of a communications system which requires the reliable transmission of data between subscribers and which further provides the control of switching paths between various terminals. The telephone switching network, as indicated, is a means by which a communication channel capable of carrying information between any two subscribers is established and maintained. As is well known, such systems consist of many intricate details which are combined into an overall system capable of operating according to relatively broad principles. It is the nature of a switching system to resolve many problems which arise from the use of a telephone instrument by a subscriber, as well as problems associated with the use of electromechanical and other devices.
A basic requirement of any communications system, including a telephone switching system, is the reliable transmission of status information in order to properly control connections between subscriber lines and so on. In other words, a receiver in such a system must know the exact nature of the transmitter before the system can properly interpret and use the data being received. For example, in a telephone switching system when a user desires to make a call, he lifts his handset associated with the telephone instrument off hook. This off hook status informs the system that a service request is desired and a subscriber desires to make a call and subsequently be connected to a particular called party.
Thus, the on/off hook condition of a subscriber is a status condition which operates to inform the switching network that a service request is desired. Accordingly, other examples of status signaling in a telephone switching network include dial pulsing and ringing, as these conditions inform the switchboard that a line is either being used or a connection to a line is to be made and hence, the switching system must recognize the status of each line according to such condtions and keep the line in a busy state until it again becomes available.
In regard to status signaling, there are two major problems usually encountered in providing and establishing reliable status communication. A first problem is associated with the telephone subset and relates to the on/off hook condition. Most telephone subsets employ an electromechanical switch or device which is typically associated with switch bounce or contact bounce. In an all relay switching system, contact bounce can be discriminated against or even ignored as when a subscriber goes off hook, a central office battery causes a current to flow through the subset and the flow of such a current will activate an electromechanical relay or other device indicating that the subscriber has gone off hook. However, the modern switching systems employ high speed digital techniques and based on digital circuitry and high speed operation, switch bounce from electromechanical transducers create transient states which appear as improper data or in general, serve to confuse the high speed digital receiving systems incorporated in the digital switching network.
Apart from the switch bounce problem, a communication line which carries digital data may be interrupted, broken or otherwise interfered with, which conditions can also lead to false signaling and hence, provide the digital receiver with false data before the alarm circuitry associated with the system can implement appropriate preventive measures.
In any event, a widely used system for digital switching networks is referred to as a time division multiplex system (TDM). In such systems, each subscriber in a group can convey and receive digital data in a time slot or channel associated with the subscriber. TDM systems are well known in the telephone switching art and reference is made to a text entitled "Transmission Systems for Communications" by the Bell Telephone Laboratories, Third Edition (1964).
In prior art digital time division multiplex systems, contact bounce or switch bounce is discriminated against by the use of a low pass filter or some other debouncer circuitry. Essentially, a suitable circuit configuration consists of an R-C low pass filter for each line circuit, which filter is coupled between the electromechanical state detector and the multiplexer in the transmitter. Essentially, the prior art is replete with various schemes for discriminating against contact bounce and such techniques have been widely employed and utilized in the past. However, the prior art techniques require a separate debouncing circuit or a separate filter for each input channel and hence, for each subscriber line. As one can ascertain, while the solution appears to be relatively simple, the cost of implementing the same can be extremely expensive when considering a large multichannel system.
In regard to the problem of data interruption as affecting the communications line, prior art systems employed a network of shift registers at the receiving terminals, which registers operate to hold the signaling information or the status information for a predetermined period, which period is long enough to allow the system line failure detectors to determine a false signaling mode and attempt to compensate for the failure before false data is released from the receiver to the local terminal associated with the switching network.
It is an object of the present invention to provide a digital debouncing and storage circuit, which is incorporated at a receiving location in a TDM switching system and which circuit automatically provides a proper signal delay, while discriminating against switch or contact bounce for each of a group of subscribers associated with a sampling multiplexer.
The apparatus to be described essentially combines three basic functions in a single circuit configuration which enables a significant reduction in cost, parts count and space requirements, while providing and assuring reliable signaling data transfer for the network control associated with the switching system.
It is a further object of the present invention to provide a method for extracting signal status information from a multiplex signal, where the signal status information is undesirably effected by switch and contact bounce.